This invention relates to a high performance exhaust system for a multi-cylinder internal combustion engine and more particularly to an exhaust system that permits a higher specific output and also which improves the output of the engine at mid-range and idle running conditions.
It is well known that the power output of an internal combustion engine, at any particular running condition, is determined by the amount of fuel/air charge that can be successfully inducted into the combustion chamber and completely burned during each cycle of the engine operation. Thus, the efficiency of the engine is directly related to its charging efficiency. A wide variety of devices have been employed for improving the charging efficiency of an internal combustion engine. Such devices include multiple intake and exhaust valves, turbo-charging, and/or the use of considerable overlap between the opening of the intake valve and the closing of the exhaust valve. High performance engines normally include either valve or port timing (depending on whether they are four or two-cycle type) that have considerable overlap to achieve high power output.
Although such overlapping valve or port timing is very effective to improve the high performance output of an internal combustion engine, such an arrangement for increasing the power output significantly reduces the performance at mid-range conditions, particularly when several cylinders of the engine discharge into a common exhaust device such as an expansion chamber. The reason for this is that there will exist at the exhaust port of the engine a high pressure during a stage of the engine operation when the intake valve is also opened. This high pressure may be caused from the transmission back to the exhaust port of a pressure pulse in the exhaust system. Such pressure pulses may be transmitted from other exhaust ports back through the expansion chamber. Therefore, rather than drawing a fresh fuel/air charge into the combustion chamber through the intake port, the exhaust gases tend to flow back into the combustion chamber through the exhaust port. This not only dilutes the fresh fuel/air charge in the combustion chamber but it also precludes the introduction of a complete fuel/air charge. As a result, many high performance, multiple cylinder engines employing large degrees of valve overlap have extremely poor mid-range or low speed running characteristics. This manifests itself in the torque curve of the engine wherein, although maximum power output is achieved, the torque output of the engine at mid-range and low speeds is considerably poorer than a more convention engine having less valve overlap or port timing overlap.
It is, therefore, a principal object of this invention to provide an arrangement for a multi-cylinder internal combustion engine that will permit the achievement of high power outputs but which will not adversely affect idle and mid-range running.
It is a further object of this invention to provide an arrangement for a multi-cylinder internal combustion engine wherein the power output of the engine may be improved at all running conditions.
It is a still further object of this invention to provide an exhaust system for a multi-cylinder internal combustion engine that permits the use of large valve or port timing overlap without adversely affecting the performance of the engine at low and mid-ranges.